1. Field of the Invention
The present invention relates to an apparatus and method for measuring micro mass using an oscillation circuit, and more particularly, to an apparatus and method for measuring mass of biomolecule such as DNA and protein by oscillating a cantilever using an oscillation circuit.
2. Description of the Related Art
In a mass micro-balancing technology, variation of a resonance frequency of a micro matter according to a variation of mass is first measured, and the varied mass is measured based on the varied resonance frequency.
A typical method for measuring micro matter using such a mass micro-balancing technology is a quartz crystal mass micro-balancing (QCM). Sauerbrey systematically theorizes a relationship between variation of resonance frequency and increase of mass in the QCM. According to the theory of Sauerbrey, the resonance frequency is linearly reduced as the mass is increased.
The QCM is generally used to measure mass per unit area of a subject (to-be-measured). The QCM determines increased mass according to variation of a shear mode of quartz oscillator. The QCM has advantages that it is designed to make it easy to input/output a signal and excel in sensitiveness. However, the mass must be evenly distributed on a surface of the quartz oscillator and a shear mode having a high resonance frequency must be used.
As the mass micro-balancing technology, a method using light and a method using piezo-resistance have been proposed.
U.S. Pat. No. 5,719,324 discloses a sensor for measuring micro mass using light.
As shown in FIG. 1, the sensor disclosed in the Patent comprises a cantilever 12, a piezoelectric element 10 supporting the cantilever 12, and a laser diode 19 formed on a tip portion of the cantilever 12 to eradiate laser beams 20. The piezoelectric element 10 is excited by a pulse wave from an oscillator 14, thereby exciting the piezoelectric element 10.
When the subject is disposed on the cantilever 12, the cantilever 12 is deformed by the mass of the examining subject. As this point, the laser beams 20 eradiated from the cantilever 12 are detected by a photodetector 27 having first and second cells 23 and 29. The deformation of the cantilever 12 is measured based on an amount of light detected by the photodetector 27, thereby determining varied mass. The reference numerals 30 and 34 indicate counting circuits, the reference numerals 36 and 37 represent differential circuits, and the reference numerals 38 and 39 indicate signals outputted from the differential circuits 36 and 37.
However, the method using the light requires an accurate position control of the laser diode and the photodetector. Therefore, a separated position controller controlling the position of the laser diode and the photodetector is required. In addition, since a member for oscillating the cantilever and a member for measuring mass are formed independent from each other, a volume of the apparatus is increased, complicating the structure.
In the method using the piezo resistance, a piezo resistance material is doped in the cantilever to increase the mass of the cantilever, causing the cantilever to be deformed by the increased mass. The deformation of the cantilever causes the resistance to be varied. The mass is measured by measuring output voltage varied by the resistance variation.
However, in the above-described methods, since the cantilever is not actively oscillated, the variation of the cantilever is too small to precisely measure micro mass. Furthermore, a variety of periphery devices such as a device for measuring varied resistance, a device for comparing an input signal for oscillating with an output signal of a resonance frequency varied by the increased mass, and the like are required, increasing the manufacturing costs and volume of the apparatus.